In general, a suspension apparatus of a vehicle includes a shock absorber and a spring member disposed between sprung and unsprung members of the vehicle. The spring member generates elastic force, and the shock absorber generates damping force. The damping force damps vibrations between the sprung member and the unsprung member caused by approaching and separating motions between the sprung member and the unsprung member.
There has been known an electrical shock absorber in which a motor is used. This electrical shock absorber includes a motor which is rotated by approaching and separating motions between sprung and unsprung members which approach and separate from each other; and an electric circuit which connects two electric terminals of the motor so as to cause current to flow through the motor. When the motor is rotated by an approaching motion or separating motion between the sprung member and the unsprung member, an induced voltage appears between the two electric terminals of the motor, and an induced current flows through the motor and the electric circuit. As a result of the induced current flowing through the motor and the electric circuit, there is generated a motor torque which acts in a direction opposite the rotation direction of the motor. Such motor torque is utilized as damping force against the approaching and separating motions between the sprung member and the unsprung member.
An electrical shock absorber described in Japanese Patent Application Laid-Open (kokai) No. 2009-257486 includes a motor, a ball-screw mechanism, and an electric circuit. The motor and the ball-screw mechanism are interposed between first and second members. The ball-screw mechanism extends or contracts as a result of an approaching or separating motion between the first member and the second member, converts the approaching or separating motion to a rotational motion, and transmits the rotational motion to the motor. The electric circuit is connected to two electric terminals of the motor in order to cause an induced current to flow through the motor. The electric circuit includes a resistor element, an inductor, and a capacitor. By use of these elements, the induced current flowing through the electric circuit and the motor is set. A damping force (motor torque) corresponding to the set induced current is obtained.